1. Field of the Invention
The present invention relates to an elastic wave filter, for example, an (SAW: Surface Acoustic Wave) filter.
2. Description of the Related Art
A SAW device makes good use of surface acoustic waves and by disposing electrode fingers called an IDT (interdigital transducer) on a piezoelectric substrate, and performing an electromechanical interconversion between an electrical signal and an elastic wave, to have frequency selection (band filter) characteristics. The SAW filter, one of the SAW devices, is used as a band pass filter for various communication devices such as a cellular phone, in which sophistication in performance and miniaturization have been proceeding. In recent years, with advances in speeding up and enlarging capacity of wireless data communication, demands for a smaller insertion loss (attenuation of output power to input power), excellent filter characteristics in frequency selectivity, a wider band width, flatness, and miniaturization have been growing. In order to satisfy these demands, it is advantageous to use, for example, a tapered IDT filter.
FIG. 18 shows a filter 100 being a tapered IDT filter having a general configuration. The filter 100 is provided with an input side tapered IDT electrode 102 and an output side tapered IDT electrode 103, which are tapered electrodes formed on a piezoelectric substrate 101, and is configured so that elastic waves propagate from an input side tapered IDT electrode 102 side toward an output side tapered IDT electrode 103 side. A shield 104 to suppress coupling between the electrodes 102 and 103 is provided between these electrodes 102 and 103, and the shield 104 is formed as a square-shaped planar metal film (what is called a solid film). Note that the tapered IDT electrode is sometimes called an inclined interdigital electrode, a SFIT, a slanted electrode, or the like.
The electrodes 102 and 103 each are composed of two parallel bus bars 105 having a plurality of electrode fingers 106, in the respective electrodes 102 and 103, the electrode fingers 106 connected to the bus bars 105 are faced, and further, for example, two of the electrode fingers 106 are paired, and the pairs extend alternately therefrom to be a comb shape, thereby being configured as a SPLIT electrode.
In each of the electrodes 102 and 103, the electrode fingers 106 are formed so that width dimensions of the electrode fingers 106 become constant and distance dimensions between the electrode fingers 106, 106 also become constant in a propagation direction of an elastic wave. An arrangement pattern composed of widths of the electrode finger 106 and distances between the electrode fingers 106, 106 is designed so that a cycle unit λidt having a certain length is repeated. In this example, the single cycle unit λidt is constituted by the four electrode fingers 106 and distance regions between the electrode fingers 106. In this filter 100, an elastic wave having a wavelength the same as the cycle unit λidt in length propagates from the input side tapered IDT electrode 102 toward the output side tapered IDT electrode 103. From the bus bar 105 on one side toward the bus bar 105 on the other side in a direction perpendicular to the propagation direction of an elastic wave, the length of the cycle unit λidt is designed in a manner that it is gradually widened, in other words, the width of the electrode finger 106 and the distance between the electrode fingers 106, 106 are gradually widened respectively. Note that the configuration of the IDT electrode will be described in detail in embodiments, and therefore, it is not described particularly here and the drawing is also shown in a simplified manner for convenience sake, and the width of the electrode finger 106 and the width between the electrode fingers 106, 106 are shown in a constant manner.
By configuring the tapered IDT by gradually widening the arrangement pattern of the electrode fingers 106 in the manner described above, elastic waves ranging from a high frequency corresponding to a region where the cycle unit λidt is narrow to a low frequency corresponding to a region where the cycle unit λidt is wide propagate in the filter 100. By applying the above configuration, widening a band of the filter 100 is achieved.
By the way, an appearance of an SAW filter more excellent in selectivity has been expected strongly, and there have been considered various methods achieving steepness of a filter characteristic in a frequency region from a pass band to a stop band. In the above SAW filter 100 particularly, there exists a problem that an attenuation characteristic at a high frequency side (high band side) deteriorates compared with that at a low frequency side (low band side), and therefore, improvement thereof has been required.
The cause of the above-described deterioration of the attenuation characteristic will be explained briefly. When a propagation path where the cycle unit λidt corresponding to a wavelength of an elastic wave is formed is called a track, the same excitation factor exists in the tracks from a low frequency to a high frequency, which form the pass band, in regions where the electrode fingers 106 are formed in the IDT electrodes 102 and 103 in the SAW filter 100. That is, propagation states (propagation velocities) of elastic waves are the same in the regions where the electrode fingers 106 are formed. However, difference in the propagation state is generated between a region where the electrode fingers 106 are not formed and the region where they are formed, and therefore, as shown by L0 in the drawing, for example, when an elastic wave passing through a predetermined track in the input side tapered IDT electrode 102 is radiated from an end edge of the input side tapered IDT electrode 102 on the output side tapered IDT electrode 103 side to the output side tapered IDT electrode 103, the above elastic wave is refracted and is not incident on a corresponding track in the output side tapered IDT electrode 103, resulting that energy loss is caused. Due to the energy loss, the deterioration of the attenuation characteristic is caused at both the high band side and the low band side.
Then, the cycle unit λidt of a track where a high frequency is propagated is formed smaller than that of a track where a low frequency is propagated. Thus in the case of seeing propagation distances L between the input side IDT electrode 102 and the output side IDT electrode 103 respectively from an elastic wave passing through an elastic wave propagation region Tr1 where λidt is constituted narrowly at the high frequency side and an elastic wave passing through an elastic wave propagation region Tr2 where λidt is constituted widely at the low frequency side, which are shown in the drawing, for example, the propagation distance L becomes longer when seen from the elastic wave passing through Tr1. When the propagation distance L becomes long as described above, the energy loss caused by the refraction is likely to occur.
Further, the elastic wave radiated from an end portion of the input side tapered IDT electrode 102 is diffracted, and therefore, loss based on the diffraction is caused in energy propagation between the input side tapered IDT electrode 102 and the output side tapered IDT electrode 103, and the longer the propagation distance L is, the more the diffraction loss is increased. Due to these reasons, the deterioration of the attenuation characteristic at the high band side becomes larger than that at the low band side.
By the way, Patent Document 1 discloses an SAW filter configured so that a grating structure reflecting an SAW having a predetermined frequency is disposed between a tapered input side IDT electrode and a tapered output side IDT electrode at predetermined distances and a frequency stop band is formed in a pass band, in other words, a notch filter characteristic is obtained. Further, Patent Document 2 discloses that a grating reflector is provided between an input side IDT and an output side IDT to match a reflection band width of the reflector with a frequency band where a spurious signal is generated thereby suppressing a spurious signal. However, Patent Document 1 and Patent Document 2 do not disclose the above-described problems, and accordingly, these problems cannot be solved.    [Patent Document 1] Japanese Patent Application Laid-open No. Sho 61-289714 (FIG. 4, FIG. 5, and so on)    [Patent Document 2] Japanese Patent Application Laid-open No. Hei 8-335848 (FIG. 1, paragraph 0027, and so on)